1. Field of the Invention
This invention relates to the field of data communication equipment (DCE), and in particular to automatic number identification system for data access arrangement (DAA).
2. Background Art
Telephone systems were originally designed for voice communication. As new technologies have emerged, methods have been devised to transmit other types of information over telephone lines. These methods typically require the connections of other equipment besides the voice telephone set to the telephone line. Examples of such equipment include computer modems, facsimile ("fax") machines, answering machines, voice mail systems, phone patches, and automatic number identification (ANI) or "Caller ID" systems.
An automatic number identification (ANI) system allows a modem or a telephone to identify the caller ID signals without user intervention. Some telephones and modems are equipped with ANI capability to provide users the convenience of ANI system. However, the prior art telephones and modems with ANI capability suffer significant ANI signal attenuation and have limited system design flexibility.
A modem enables two digital electronic systems to communicate over the telephone network. A typical telephone network comprises a single twisted pair of wires (called the "tip" and "ring" lines). Digital equipment systems, on the other hand, require two separate paths of communication to communicate with each other. Also, the signals present within the above equipment often have different electrical characteristics than the signals that may be transmitted on a telephone line. Thus, an interface is required to connect such equipment to a telephone line.
The modem is placed between the digital computer and the analog telephone system, providing the necessary interface between the telephone network and digital systems over the tip and ring lines. The modem accepts a serial stream of bits as input and produces a modulated carrier as output, thus converting the digital signals of the computer to analog signals for transmission on telephone lines, or vice versa.
To meet regional telephone companies' requirements, a modem typically comprises a DAA (Data Access Arrangement), which is used to connect to a dial-up switched telephone line. Traditionally, especially for computer modems, a DAA is used to connect equipment to a telephone line. A DAA typically provides isolation, impedance matching, hybrid circuit and sometimes amplification, filtering and control functions.
When a modem is equipped with an ANI system, it allows to identify the caller ID and register the caller ID in a memory device. If the modem is connected to a computer, then other options are possible to use the caller ID in more sophisticated ways. For example, the computer can receive the caller ID from the modem, display it on a screen, store it in its memory, or write it in a file.
An ANI system is also useful for such state-of-the-art technology as "recall." When an attempted telephone call goes unanswered, the caller ID is identified by an automatic number identification system on the called telephone and stored in a memory device such as a random access memory (RAM). When a user on the called telephone side wishes to call the last number the user missed, the user can press a special "recall" button or a combination of buttons, such as the "*" key followed by the "9" key, on the user's telephone key panel to initiate an outgoing call to the last number that called. Upon detecting a special key sequence for "recall," the user's telephone makes a "recall" to the telephone number that last called.
In an ANI system, caller ID signals are sent to a called modem or a telephone when a call is made to the called telephone number. The caller ID signals provide the called telephone or modem with identification of the calling telephone. The called telephone uses the caller ID (ANI) signals to identify the calling telephone or modem. Typical caller ID signals include frequency shift keyed (FSK) modem tones transmitted between rings of the ringing signal.
Prior art ANI systems were implemented by adding extra circuit components to DAA circuits to implement a caller ID signal path. These prior art ANI systems often led to a more complex, expensive design architecture, and resulted in significant caller ID (ANI) signal attenuation up to 18 dB or more. FIG. 1 shows a block diagram of a prior art DAA circuit for a European-compatible modem with ANI capability. The circuit in FIG. 1 has a caller ID signal path that comprises switch SW3, capacitor C2, and resistor R1. Capacitor C2 is used to block DC current from being supplied to transformer T1.
In FIG. 1, transmitted signal TXA is amplified by amplifier A1 and provided to transformer T1 through impedance network Z1. The typical value of Z1 is approximately 300 .OMEGA. for U.S. designs and 300 .OMEGA.+(374 .OMEGA. resistance in parallel with 200 nf capacitance) for international designs. Received signals from the telephone line (represented by the tip and ring lines) are fed to amplifier A2 and to line RXA. Transformer T1 isolates the outgoing signal from the received signal. Transformer T1 has a primary winding and a secondary winding.
The primary winding of transformer T1 is coupled to capacitor C1, which is coupled to the "tip" line through switch SW1. Protection circuit PC1 comprising Zener diode ZD1 and diode bridge DB1 (not shown in detail, well-known in the art) is coupled across the primary winding of transformer T1 via capacitor C1. Protection circuit PC1 is used to protect the modem from voltage spikes and to ensure proper operation of a DAA circuit since the polarity of tip and ring lines are not known in advance. The primary winding of transformer T1 is also coupled to the "ring" line. The tip and ring lines (local loop) are connected to the telephone central office that provides power, switching, and signaling.
On a local loop represented by the tip and ring lines, an idle (on-hook) state is indicated by an open circuit and no current flow. A connect (off-hook) state is signaled by a closed circuit and continuous current flow. In an off-hook state, the telephone line is said to be "seized." Referring to FIG. 1, during the idle state (on-hook) before the modem has seized the line, an incoming ring signal is detected by the ring detector block (not shown, well known in the art) and switch SW3 is closed.
The caller ID signals (ANI information) are transmitted from the telephone system central office to the local DAA, and usually come between the first and second incoming rings in the form of a small AC signal. The modem can detect the caller ID signals at the RXA line if an additional AC coupling path is provided.
The additional AC coupling path for the caller ID (ANI) signals is provided in the prior art example of FIG. 1 along switch SW3, capacitor C2, and resistor R1. The existing path along switch SW1 and capacitor C1 cannot be used as the AC coupling path for the caller ID (ANI) signals since capacitor C1 by itself would exhibit an impedance below the regulated value of two (2) K.OMEGA.. Although a resistor can be used in series with capacitor C1 to increase the impedance, the addition of such series resistor would also increase the DAA's off-hook impedance value to an unacceptable range.
Referring to FIG. 1, switch SW3 is closed for the duration of an automatic number identification process, and an AC current path is formed by components R1 and C2 to allow the caller ID (ANI) signals from the telephone line to reach transformer T1 and RXA. Switches SW1 and SW2 remain open during ANI operations so that no DC current flows between the DAA and the telephone central office, thereby preventing the telephone line seizure. After an automatic number identification process is complete and the caller ID has been detected, switch SW3 is opened. In another prior art embodiment which is compatible with the U.S. and Canadian regulations but incompatible with most European regulations, switch SW3 is not required and can be replaced with a conducting wire.
In FIG. 1, capacitor C2 in the caller ID (ANI) signal path typically has a voltage rating of at least fifty (50) volts to handle the local loop voltage (typically forty eight volts) and thus is limited below a maximum capacitance value. Capacitor C2 has the typical capacitance value between 0.15 .mu.f and 0.47 .mu.f. Resistor R1 is used to ensure that the impedance during an ANI operation is not lower than a regulated value on the order of two (2) K.OMEGA.. Resistor R1 has the typical resistance of 2 K.OMEGA..
A combined effect of C2 being limited in capacity and having resistor R1 is the attenuation of the caller ID (ANI) signals of more than eighteen (18) dB when the caller ID signals reach the RXA line. Consequently, the caller ID (ANI) signals that appear at node N1 are significantly attenuated and become susceptible to noise and error.
In an off-hook (connect) state, the modem closes switches SW1 and SW2, seizing the telephone line (represented by the tip and ring lines) and activating the DC portion of artificial inductor AI1. Transmitted signals that come in from the TXA line are modulated signals, i.e, carrier signals modulated by digital signals that originate from a digital data terminal equipment (DTE) such as a personal computer using a suitable modulation technique. Typical modulation techniques include amplitude, frequency, and phase modulations. Thus, signals received from the tip and ring lines are modulated signals. Transformer T1 couples these modulated carriers in both directions: from the telephone line to received signal (RXA) and from transmitted signal (TXA) to the line in the connect state. This composite AC signal comprising transmitted and received signals flows through coupling capacitor C1. Capacitor C1 has the typical capacitance value of 3 .mu.f-5 .mu.f.
In the off-hook (connect) state, switch SW2 is closed and suitable protections are provided by protection circuit PC1 against voltage spikes and polarity reversal. SW2 is a required component for European-compatible modems for pulse dialing. In an alternate prior art design, SW2 is not required and can be replaced by a wire, for example, for U.S. or Canadian design. During pulse dialing, SW1 is closed and SW2 is pulsed to generate dial pulses consisting of current flow interrupted at a specified rate in the loop current (between a local DAA and the central telephone office). Zener diode ZD1 provides overvoltage protection by absorbing the voltage transients generated during the pulsing.
FIG. 2 shows another prior art DAA system for a European-compatible modem with ANI capability. In FIG. 2, transformer T1 is coupled to the DC side ("+" and "-" terminals) of diode bridge DB1 via capacitor C1. This embodiment allows capacitor C1 to be a smaller polarized capacitor (positive plate denoted by "+" sign) as shown in FIG. 2 rather than a larger unpolarized capacitor of FIG. 1. In an ANI operation, switches SW3 and SW4 are closed to form a circuit path for the caller ID signals. The approach shown in FIG. 2, however, requires an extra switch element SW4 to return the bottom leg of the transformer to the correct node in addition to the caller ID signal path comprising resistor R1, capacitor C2, and switch SW3.
FIG. 3 shows a plot of input impedance versus ANI signal frequency for DAA circuits shown in FIGS. 1 and 2. In FIG. 3, input impedance Zin (assumed to be resistive) looking into the modem from the telephone line varies from about 2.3 K.OMEGA. to 3.9 K.OMEGA. depending on the signal frequency.
FIG. 4 shows the frequency response of ANI signal gain for DAA circuits shown in FIGS. 1 and 2. In FIG. 4, the vertical axis shows ANI signal gain in dB at RXA line (pin). As shown, in the prior DAA circuits, the caller ID (ANI) signals arriving at node N1 are significantly attenuated due to a voltage divider effect with Z1 on the bottom of the divider and C2+R1 (plus transformer T1's winding resistance) on the top, with node V1 at the midpoint. Typically, ANI signal attenuation in the prior art DAA circuits ranges from 18 dB to 31 dB, significantly weakening the caller ID (ANI) signals.
Thus there is a need in the art to provide a DAA with ANI capability that reduces the caller ID signal attenuation without requiring significant modifications to the existing DAA designs and without greatly increasing design complexity.